The present invention is directed to backlighting systems and more particularly to high brightness homogeneous intensity backlighting systems for use with one or more light source such as light emitting diodes, fluorescent lamps and lasers.
Backlighting systems are presently employed for a variety of display purposes such as for personal digital assistants (PDAs), car audio components, displays and illumination and cell phones. Backlighting increases the effectiveness of a display by better contrasting display graphics against a brighter background. Performance of a backlighting system is based upon criteria including brightness, apparent homogeneous light distribution, directional viewing zones and power usage. The latter characteristic is especially important in portable, battery-operated applications such as PDAs and cell phone displays where power usage directly affects battery life. As displays for PDAs and cell phones become more exotic with the addition of multi-color graphics and better resolution, the need for high brightness, homogeneous and lower power usage backlighting systems has become more extreme.
Typical prior art backlighting systems employ a bright light source such as a fluorescent lamp. Unfortunately, such lamps require complex optical couplers to direct the light in only a desired direction such as into a light pipe or optical wave guide exhibiting total internal reflection (TIR). Such lamps also suffer the disadvantages of relatively high power usage, low durability and short life span and lose energy at a rate of 1% per 100 hours of usage. In addition, it is often difficult to couple the light to the light pipe without creating dark zones or lines which detract from the desired homogeneity of the backlighting and the aesthetics of the display.
It would therefore be highly desirable to provide a backlighting system which provides solutions to these disadvantages by overcoming the conventional coupling requirements, dark zones, high power usage and life span limitations of the prior art.
The preferred embodiment of the present invention comprises an LED-based backlighting system that provides excellent brightness and homogeneity suitable for high performance multi-color and high resolution portable displays such as for PDAs and cell phones. The invention herein is also advantageous for use with other light source fluorescent lamps, as well as coherent and non-coherent light. Several different but similar embodiments are disclosed. Some are suitable for use with multiple LEDs and some for use with a single LED and some with fluorescent and other light sources. Each exhibits superior brightness and each solves the problem of hot spots or dark areas that can detract from achieving a homogeneous distribution of light intensity over the entire backlight region. Moreover, because each such embodiment is specifically designed to operate with one or more LEDs as a light source, no coupler is required, life span is greater, durability is greater and power usage is reduced.
In a first such embodiment, multiple LEDs are located adjacent the input edge of a light pipe. The input edge contains fixed pitch grooves to propagate the LED light in a direction that is substantially angled toward the bottom surface of the light pipe along its entire area. The light pipe bottom surface may have grooves for altering the direction of the LED light toward the upper surface of the light pipe where it exits the light pipe toward a viewer. The light pipe bottom surface may also be formed from silk screen dot patterns or a holographic surface structure. The exterior of the exit surface of the light pipe receives a light shaping diffuser (LSD) and other diffuser or optical film. The diffuser acts to diffuse and distribute the exiting light at selected horizontal and vertical angles and the optical film shapes the light intensity distribution to achieve optimum brightness toward the viewer.
In a second embodiment, a second LSD is positioned above the optical film. In a third embodiment, an LSD strip comprising a foreshortened LSD is positioned below the light pipe adjacent the input edge to better reduce hot spots near the LEDs. In a fourth embodiment a second LSD below and contiguous to the light pipe is used instead of the aforementioned bottom surface grooves. In a fifth embodiment a small LSD replaces the fixed grooves at the input edge. In a sixth embodiment where only one LED is employed at an input corner of a grooved light pipe, a fixed angle or variable angle LSD is used at the upper surface of the light pipe. In-a seventh embodiment, which is also a single LED version, a second fixed angle LSD is added above the variable LSD of the sixth embodiment. Such LSD may also be injection molded as an integral part of the backlight optical film. The light pipe of the sixth and seventh embodiments employs circular-shaped grooves. An important feature of the light shaping diffusers of the present invention is their directionality, controlled scattering, homogeneous characteristic and seamless variation of diffuser angle which promotes a homogeneous backlight distribution without disruptive lines.